I found Purity, which revolves around people who have variously relegated themselves to the fringes of society, immensely alienating.
This book delivers a great strike against the notion that the idealist is an exceptional individual, someone more enlightened, more autonomous, more principled than the droning masses, by showing how self-deluding and reactive the uptake of countercultural values can be.

I haven't read Purity, but I have read (and loved) other Franzen novels. Sounds like this is your first? Based on your description, I think you'll like his other work. The Corrections and Freedom are great, and I hear great things about Crossroads too. His essays are also good; I love the one about birdwatching.
I like when authors are essayists also :) mentally filing this tidbit away for my commute